Control and Design of Microgrid Components - Power Systems ...

Chapter 4. Interface of Inverter to Local System
This chapter describes how the ratings of the several components within a microsource need to
be coordinated to achieve the desired operation. Figure 4.1 shows the full layout of every
component that appears in a microsource: from left to right, there is the prime mover responsible
to generate a DC voltage, then there is the DC bus that needs to include some storage. The
inverter is the interface between the DC bus and AC system and is responsible for the operations
of the microsource. Immediately connected to the inverter terminals there is an L-C filter bank to
eliminate the higher harmonic from the voltage waveform and then there is the inductor that
determines how active and reactive power can be dispatched. The final connection with the
feeder is achieved by means of a delta-wye transformer with a neutral connection to allow for
single phase loads to be connected to the system.
Prime Mover
DC interface
+
Voltage Sourced Inverter
L
Filter
Inductor
X
Transformer
4 Wire
Feeder
Prime
Mover
N
C
Filter
Filter
Figure 4.1 Microsource Component Parts.
All these components need to be coordinated and selected in such a way that their ratings are
compatible with the capabilities of the other components of the chain.
4.1 Prime Mover Dynamics and Ratings
The prime mover is the block that converts the chemical energy of the fuel in DC electric power.
Some examples of prime movers are microturbines and fuel cells. Each prime mover has a
dynamic response to changes in power command that depends on the technology adopted. In
general the dynamic response takes from few seconds to few minutes to track a step change in
the power command, for example Capstone microturbine shows a time constants in the order of
10-20 seconds to follow a step change in the power command in Figure 4.2, while a fuel cell
stack takes minutes to track the power command. There is the need to provide some form of
energy storage to ensure that the energy for the loads is immediately available. The storage will
be located at the DC bus. The rating on the prime mover is given in terms of maximum active
power output, which is the largest amount of power that the generator can convert to electric
form. This is an upper limit that will influence the choice of the ratings for other components of
the microsource such as the inverter and the transformer. Usually, the prime mover has also a
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